This invention pertains to cochlear electrode arrays shaped to a predetermined curvature, and more particularly to a cochlear electrode array which has a first preselected shape suitable for insertion into the body of a patient, and a second shape suitable for providing a specific function, or stimulus.
Cochlear implant systems are used to aid patients having a hearing deficiency. More particularly, these systems include a microphone receiving ambient sounds and converting the sounds into corresponding electrical signals, signal processing means of processing the electrical signals and generating cochlea stimulating signals and an electrode array for applying the cochlea stimulating signals to the cochlea of the patient. It is known in the art that the cochlea is tonotopically mapped. In other words, the cochlea can be partitioned into regions, with each region being responsive to signals in a particular frequency range. This property of the cochlea is exploited by providing the electrode array with a plurality of electrodes, each electrode being arranged and constructed to deliver a cochlea stimulating signal within a preselected frequency range to the appropriate region. The electrical currents and electric fields from each electrode stimulate the auditory nerve cells disposed in the modiolus of the cochlea. Several electrodes may be active simultaneously.
It has been found that in order for these electrodes to be effective, the required magnitude of the currents flowing from these electrodes is a function of the distance between the electrodes and the modiolus. If this distance is great, the threshold stimulation current magnitude must be larger than if the distance is smaller. Moreover, the current from each electrode may flow in all directions, resulting in the area of the cochlea stimulated by a single electrode being undesirably large. Therefore the electrical stimulation is not well localised to a particular site on the cochlea. In order to reduce the threshold stimulation amplitude and to improve localisation, it is advisable to keep the distance between the electrode array and the modiolus as small as possible. This is best accomplished by providing an electrode array having a shape which generally follows the shape of the modiolus. In contrast during insertion, the electrode array should be generally straight, because otherwise the insertion procedure is too cumbersome and difficult. Consequently there is a problem due to the hitherto conflicting design objectives that the electrode array be straight during insertion but curved during use.
Several methods and means of curving the electrode array and therefore overcoming the above-described problem have been attempted. These attempts fall generally in two categories. The first category consists of arrays that are formed in a straight configuration, and are mechanically manipulated into a curved configuration by an external device which exerts pressure against the outside wall of the cochlea. These arrays are designed so that part of the array is pressed against the outside wall of the cochlea, and another part is thereby pressed against the inside wall. These types of arrays may be of a two-part design (such as commonly assigned U.S. Pat. Nos. 5,645,585 and 5,545,219,) or they may be of a space-filling design. Both share the disadvantage of exerting a permanent pressure against both the inside and outside wall of the cochlea. The space-filling designs have an additional disadvantage that they displace the cochlear fluid, which may have adverse affects on the patient.
The second category consists of arrays which are shaped into a curved configuration and are then straightened for insertion. Examples of arrays falling into this second category include an electrode array having an electrode carrier provided with a longitudinal element arranged on one side of the carrier which is constructed to change its dimension once the array is inserted. For example, the longitudinal element could include a hydrogel such as PAA (Polyacrylic Acid) which expands after insertion by absorbing water from the cochlear fluid, as described in commonly assigned U.S. Pat. No. 5,578,084. Alternatively, the longitudinal element could be a bimetallic filament (such as nickel/titanium filament) which is shaped to allow the electrode carrier to take a straight configuration at room temperature but bends into a preselected shape once it is exposed to body temperature.
Commonly assigned U.S. Pat. No. 5,653,742 discloses another electrode array falling into the second category. In this patent, the array is encapsulated into a stiffening sheath which holds the array in a linear configuration. The sheath is made of a biosorbable material such as polyvinyl alcohol (PVA) which dissolves in the cochlear fluid after insertion.
While the arrays from this second category remove the disadvantage of the static pressure against the walls of the cochlea, they have other disadvantages in that the surgeon cannot control the point in the surgical procedure at which the array curves, and the array cannot be restraightened during surgery to allow a second attempt at insertion if the first attempt fails.
In view of the above-mentioned disadvantages of the prior art, it is an objective of the present invention to provide a cochlear electrode array which has a first, relatively straight configuration so that it can be readily implanted, using a stylet which, after insertion is readily removed and a second, curved configuration to conform to the cochlea of a patient.
A further objective is to provide an array which is small relatively in cross-sectional area, so as to facilitate insertion and minimise the displacement of the cochlea fluid.
Another objective is to provide an array which can be manufactured relatively easily and inexpensively.
Other objectives and advantages of the invention shall become apparent from the following description.
Briefly, an electrode array in accordance with this invention includes an elongated carrier which supports a plurality of electrodes suitable for defining an electrode array for application for cochlear stimulation signals, and wires attached to the electrodes to provide cochlear stimulation signals in the usual manner. The carrier is preferably preshaped into a curved configuration selected to insure that the electrodes are disposed in close proximity to the modiolus of the scala tympani. Importantly, the carrier is formed with a lumen which is designed to accept one or more surgical stylets. Prior to insertion of the carrier into the cochlea, the stylet is introduced into the carrier to insure that the carrier is maintained in a substantially straight configuration. As the array is inserted into the cochlea, the stylet is slowly withdrawn allowing the array to assume a curved configuration. Preferably the stylet is relatively stiff along its entire length except for its tip. The tip is annealed to render it more malleable then the rest of the stylet to allow the array to flex easily as it is being inserted.
Preferably, the electrodes are disposed on an inner surface of the carrier, so that when the carrier is implanted, the electrodes are facing the modiolus. Each electrode may be formed from a ring with a wire threaded through the ring, the ring then being collapsed into a U-shaped electrode element to grip the wire. The electrodes and wires are then embedded into the carrier.
Several methods for producing the electrode array can be used. For example, a blank may be formed around a curved production stylet, and the stylet may be withdrawn from the blank to form a lumen. The electrodes and their wires are attached to the blank and a moulding material is applied to complete the carrier.
After the carrier and its electrodes are completed, it may be packaged in its curved configuration together with an appropriate surgical stylet, and a straightening jig. Prior to surgery, the straightening jig is used to simultaneously straighten the carrier and insert the stylet into the lumen of the carrier. The stylet is stiffer and more rigid than the carrier so that once the stylet is inserted into the lumen, the electrode array can be removed from the jig with the carrier maintaining its straight configuration for insertion.